When you open a can of baking powder, moisture from the air gets in. Moisture, usually in the form of liquid ingredients in your baking recipe, cause reactions to begin occurring between the baking soda and acid forming ingredients in the baking powder. Unfortunately, moisture from the air can cause these same reactions to occur, so that baking powder will lose its potency over time. Starches are used in baking powders to help keep this from occurring. You can read more about baking powder and how to make it homemade, here.

How Can I Test My Baking Powder to See If It's Still Good?

It is very simple to test your old baking powder for potency before using it. All you have to do is mix a small amount of the baking powder with water. The mixture should start bubbling as carbon dioxide gas is formed. This reaction will look similar to when you put Alla-Seltzer tabs into water. The more vigorous the bubbling, the more potent your baking powder is.

The photo below shows the bubbling action of some good, potent baking powder added to water.

The vigorous bubbling action of this tested baking
powder shows that it is good to go!

The vigorous bubbling action of this tested baking
powder shows that it is good to go!

If it doesn't react very much and the bubbling action isn't very vigorous, your baking powder has lost a lot of its potency and will not work well in your baking recipe. If you compare some very old baking powder with a brand new can by mixing some of both with water, you will undoubtedly notice a difference.